The Chauvin Arnoux PT100 standard reference or working thermometers made with sturdy platinum resistance sensors are used as definitive tools in laboratory and meteorological studies and workshops. Both offer high measurement reproduction accuracy. PT100 reference thermometers' sensors drift < 25 mK on the whole temperature range of -100 - +450 °C. Moreover, these may use either ITS-90 fixed point of comparative calibration methods. On the other hand, PT100 working thermomters' sensors drift at < 50 mK on the whole temperature range of either -100 to +550 °C or -100 to +450 °C, depending on the maximum working temperature, and they use comparative calibration methods.